1. Field of the Invention
The invention relates to an engine control system for a direct injection-spark ignition type of engine, and, in particular, to a direct injection-spark ignition engine control system for controlling an fuel injection timing while the engine is in a cold condition.
2. Description of the Related Art
Engine control system of this type incorporate in an exhaust line a NOx adsorbing type of lean NOx conversion catalyst which, on one hand, adsorbs NOx in the exhaust gas while the air-fuel mixture is leaner than a stoichiometric mixture and, on the other hand, desorbs or releases the NOx into exhaust gas for catalyzing reduction of the NOx while the air-fuel mixture is richer than a stoichiometric mixture. As is known from, for example, International Patent Application W093/07363, such an engine control system controls the engine to operate with an enriched mixture under accelerating conditions or full loading operating conditions and with a lean mixture under the remaining operating conditions, so as to improve fuel consumption.
An engine control system for a direct injection-spark ignition type of engine known from, for example, Japanese Unexamined Patent Publication 7-119507 controls the engine to cause stratified charge combustion in a lower engine loading zone and homogeneous charge combustion in a high engine loading zone. While the engine operates with lower speeds in the high loading zone, a given amount of fuel is delivered in two steps through early and late split injection in a intake stroke, so as to diffuse a first half of fuel sprayed through the early split injection in the combustion chamber before the end of a intake stroke and the second half of fuel in the combustion chamber with its volume increased, This prevents a generation of rich or dense mixture over the top of a piston in a subsequent compression stroke that generally occurs when a given amount of fuel is sprayed all at once through non-split injection, which is desirable to prevent generation of smoke.
Another engine control system for a direct injection-spark ignition engine cooperates with a fuel injector which is direct to face the top of a piston and energized to spray a small amount of fuel preparatorily at the beginning of a intake stroke when the engine causes knocking. The fuel partly sticks to the top wall of the piston on a side of an intake port and bounces off the piston wall toward the intake port to cool the piston head and the combustion chamber on the intake port side with the heat of vaporization of the fuel. Such an engine control system is known from, for example, Japanese Unexamined Patent Publication 7-217478.
Generally, while an engine operates in a cold condition, fuel sprayed in the combustion chamber assumes aggravation of evaporation performance with an effect of a drop in ignitability and combustibility, which is always undesirable for combustion stability. Because a direct injection-spark ignition type of engine in particular provides only a period of time for fuel evaporation shorter than what is called a port injection type of engine, it causes remarkable aggravation of fuel evaporation performance.
It may be effective to avoid aggravation of fuel evaporation performance in a cold engine operation to increase the amount of fuel injection sufficiently to provide a large amount of evaporated fuel even during a cold engine, so as thereby to keep aggravation of ignitability and combustibility under control and, in consequence, to preserve desired combustion stability. However, combustion of an increased amount of fuel produces an increased amount of hydrocarbon (HC) and carbon monoxide (CO) due to over enrichment of a fuel mixture as well as lowering specific fuel consumption.